Electromagnetic interference (EMI) is disturbance that can affect a communication system due to electromagnetic radiation emitted from an external source. EMI can interrupt, obstruct, degrade, or limit the performance of the communication system. The effects of EMI can range from a degradation of data to a total loss of data. The source of EMI may be any artificial or natural object that carries rapidly changing electrical currents.
EMI may be a serious concern for high speed communication systems that include long physical channels. An example of a communication system that can be affected by EMI includes a 10GBase-T Ethernet system that transfers data at a rate of 10 gigabits per second over Ethernet cables having lengths of up to 100 meters. For a 10GBase-T system having a signal bandwidth as high as 400 megahertz (MHz), EMI near 400 MHz, such as EMI emitted from a cell phone operating at a frequency of 850 MHz or a walkie-talkie operating at a frequency of 460 MHz, can severely degrade the signal-to-noise ratio (SNR) of the 10GBase-T system. A 10GBase-T system may also be adversely affected by EMI at a frequency around 800 MHz resulting from a coupling of signals between adjacent cables.
FIG. 1 is a frequency graph 100 showing examples of desired frequencies 102 and undesired frequencies 104, 106, 108 in a signal. In FIG. 1, the desired frequencies 102 represent, for example, the signal bandwidth of a 10GBase-T Ethernet communication system. In a 10GBase-T system, the signal bandwidth can be as high as 400 MHz. One or more undesired frequencies 104, 106, 108 may be injected into the communication signal from nearby sources, such as a walkie-talkie operating at a frequency of 460 MHz or a cell phone operating at a frequency of 850 MHz, or from a coupling of signals between adjacent cables of the 10GBase-T system that produces a disturbance at a frequency around 800 MHz.
FIG. 2 is a block diagram showing an example of a lowpass filter chain 200 that may be used to attenuate undesired frequencies in an input signal, e.g., the undesired frequencies 104, 106, 108 of FIG. 1, and to provide a filtered version of the input signal as an output signal. The lowpass filter chain 200 includes multiple first-order lowpass filters, e.g., LPF(1) and LPF(2) to LPF(N). N stages of first-order lowpass filters can be cascaded to implement an Nth-order lowpass filter. The frequencies of cascading lowpass filters in the lowpass filter chain 200 must align with each other so that the lowpass filters collectively have the desired frequency response. To achieve the desired attenuation of undesired frequencies that are very close to a desired frequency, a high-order lowpass filter may be needed.
FIG. 3 is a frequency graph showing examples of frequency responses 300 of the lowpass filter chain 200 of FIG. 2. As the number of stages of lowpass filters increases, roll-off of the frequency response increases which causes increasing attenuation of frequencies near and above the cutoff frequency fc. If an undesired frequency is very close to a desired frequency, the cutoff frequency may need to be near the desired frequency to sufficiently reduce the undesired frequency. When the cutoff frequency is near the desired frequency, the lowpass filter chain may also attenuate the desired frequency.
To attenuate an undesired frequency that is very close to a desired frequency, a notch filter may be used. FIG. 4 is a schematic diagram showing an example of a notch filter 400 that may be used to attenuate an undesired frequency in a signal. The notch filter 400 is implemented using passive components that do not depend on an external power supply. The notch filter 400 includes a resistor 402, an inductor 404, and a capacitor 406. The notch frequency of the notch filter 400 is dependent on the values of the resistor 402 and the capacitor 406. The notch filter 400 receives an input voltage Vin of an input signal and provides an output voltage Vout of an output signal. The notch filter 400 passes most frequencies unaltered, but attenuates a narrow band of frequencies to very low levels. To implement a notch filter that includes inductors with a high quality (Q) factor, the inductors may need to be accurately modeled and sufficiently protected from electromagnetic interferences.
FIG. 5 is a frequency graph showing examples of frequency responses 500 of a notch filter, e.g., the notch filter 400 of FIG. 4. As the Q factors of the inductors increase, the band of frequencies that are attenuated becomes smaller, and the amount of attenuation at the undesired frequency fnotch increases. In other words, as the Q factors of the inductors decrease, the band of frequencies that are attenuated become larger, and the amount of attenuation at the undesired frequency fnotch decreases. Using a notch filter that includes inductors having low Q factors may also attenuate a desired frequency, in addition to the undesired frequency, when the notch frequency of the notch filter is near the desired frequency of the signal.
FIG. 6 is a schematic diagram showing an example of a notch filter 600 that may be used to attenuate an undesired frequency in a signal. The notch filter 600 is an active twin-T notch filter. The notch filter 600 receives an input voltage Vin of an input signal and provides an output voltage Vout of an output signal. The notch filter 600 includes capacitors 602, 604, 606, resistors 608, 610, 612, 614, 616, and an operational amplifier 618.
The capacitors 602 and 604 each have a capacitance value C. The capacitor 606 has a capacitance value 2C or two times the capacitance value C of capacitor 602 or 604. The resistors 608 and 610 each have a resistance value R. The resistor 612 has a resistance value R/2 or half of the resistance value R of resistor 608 or 610. The ratio of the resistance value R of the resistors 608 or 610 to the resistance value R/2 of the resistor 612 is 2. The ratio of the capacitance value 2C of capacitor 606 to the capacitance value C of capacitors 602 and 604 is 2. The resistor 614 has a resistance value Ra, and the resistor 616 has a resistance value Rb. The resistance values Ra and Rb are typically much less, e.g., an order of magnitude smaller, than the resistance value R. The notch frequency fnotch of the notch filter 600 is determined by the capacitance value C and the resistance value R, and is defined by the equation fnotch=1/2πRC. The Q factor of the notch filter 600 is determined by the values Ra and Rb of the resistors 614 and 616, and is defined by the equation Q=Ra/4Rb. For example, a filter having a resistance value R of 100 Kohms, a capacitance value C of 1 pF, a resistance value Ra of 10 Kohms, and a resistance value Rb of 250 ohms can achieve a notch frequency of 1.6 MHz and a Q factor of 1.0.
Similar to the notch filter 400, the notch filter 600 attenuates a specific frequency of an input signal while allowing all other frequencies to pass unaltered. In contrast to the notch filter 400 of FIG. 4, the notch filter 600 does not include any inductors. The notch filter 600 passes low frequency signals unaltered through the series resistor path that includes the resistors 608 and 610, and passes high frequency signals unaltered through the series capacitor path that includes the capacitors 602 and 604. At notch frequency, the feedback from the operational amplifier 618 facilitates cancellation of signals from the series resistor path and the series capacitor path that are 180 degrees out of phase. Thus, the notch filter 600 operates without the use of an inductor.
The notch frequency fnotch of the notch filter 600 can be adjusted independently of the Q factor because the notch frequency is dependent on the resistance value R and the capacitance value C whereas the Q factor is dependent on the resistance values Ra and Rb. Such independent adjustability of notch frequency and the Q factor of the notch filter 600 eliminates the need for inductors with high Q factors. Because a notch filter attenuates only frequencies in a narrow band, additional lowpass filters may be needed to further process the signal to reduce undesired frequencies that are higher than the notch frequency of the notch filter.